Oil refineries and other industries commonly utilize underground drain lines which are in communication with ground level through one or more drain assemblies. Such drain assemblies commonly include a grating support ledge and a peripheral wall extending a predetermined distance upwardly from the grating support ledge toward ground level. In normal operation fluid-flow communication exists through a grating disposed on the ledge.
Certain operations such as welding and burning can have disastrous consequences if carried out close to open drains of the aforesaid character. Flammable materials in gaseous and liquid form are often located in the underground lines and represent fire and explosion hazards if ignited.
Process industries, such as oil refineries, are acutely aware of this problem and have taken special steps to isolate their process drains when welding or other potentially harmful operations are to be carried out.
The most common approach for isolating process drains is to cover and surround them with sandbags. This method is less than satisfactory in many instances since bags can tear and release sand which fouls and plugs the drain.
U.S. Pat. No. 3,516,541, issued June 23, 1970, discloses a drain safety device purported to overcome certain disadvantages of sandbagging of drain openings. More particularly, the device shown in such patent incorporates a rigid sheet in which a central opening is formed that is in communication with a depending cylindrical shell housed within an apertured tank secured to the sheet. The peripheral edge portions of the shell rest on body shoulders forming a part of a downwardly extending drain structure. A grated frame rests upon the upper surface of the sheet and removably holds the device in place on the upper portion of the drain structure.
After installation of the device of U.S. Pat. No. 3,516,541, water is discharged thereinto until it flows from the apertures in the tank into the drain structure, the apertures being located above the lower edge of the shell.
In essence, the arrangement of U.S. Pat. No. 3,516,541 creates a water trap or seal which would not be effective to prevent escape of drain gases which are pressurized to sufficient degree.
Furthermore, the device of U.S. Pat. No. 3,516,541 will fail as a seal entirely if the water or other liquid therein falls below the level of the cylindrical shell, as by evaporation.
With such device gases can escape from the underground line without anyone being aware of the fact. This may lull workers into a false sense of security when none in fact exists. Yet another deficiency of this prior art approach is that it requires the use of a mastic material or other sealant applied thereto to provide a seal at the location of contact between the device and the drain structure with which it is associated. The device does not lend itself to use as a temporary plug.
Other prior art structures of a similar character incorporating water traps or barriers are shown in U.S. Pat. Nos. 3,556,993, 4,682,907 and 4,776,722.